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At least 1,729 records · Page 96
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Inhibition of Chromium(III) Oxidation through Manganese(IV) Oxide Passivation and Iron(II) Abiotic Reduction

Manganese (Mn) oxides are strong oxidants that are ubiquitous in soils and can oxidize redox-active metals, including chromium (Cr). In soil environments, trivalent chromium (Cr(III)) is a benign, immobile micronutrient, whereas the hexavalent Cr(VI) form is present as a highly mobile, toxic chromate oxyanion. Although many studies have characterized the capacity of Mn(III/IV) oxides to oxidize Cr(III) to toxic Cr(VI), the oxidative capacity of Mn oxides in the presence of potentially passivating soil constituents, specifically reduced soluble iron (Fe(II) aq ), remains unresolved. We hypothesized that chemical processes at redox interfaces, such as diffusion-limited environments within soil aggregates, can lead to decreased Cr(VI) production from Mn oxide-driven oxidation due to passivation by Fe(II) aq . A multichamber diffusion-limited reactor was used to simulate transport at soil redox interfaces and investigate the capacity of poorly crystalline and crystalline Mn oxides to oxidize solid Cr(III) minerals to Cr(VI) in the presence of Fe(II) aq . As predicted, Cr(VI) was produced through the Mn oxide-catalyzed oxidation of Cr(III) at a rate controlled by the solubility of Cr(OH) 3 . However, in the presence of Fe(II) aq , the concentration of aqueous Cr(VI) decreased as a function of the Fe(II) aq concentration, where high concentrations of Fe(II) aq completely inhibited Cr(VI) production, likely through both the passivation of the Mn oxide and the direct reduction of Cr(VI) by Fe(II). At both low (14 μM) and high (100 μM) Fe(II) aq concentrations, the iron oxide minerals hematite (Fe 2 O 3 ) and goethite (α-FeOOH) were associated with the Mn oxides, which can cause surface passivation, a likely role that decreases Cr(III) oxidation. Additionally, the Cr(III) oxidation rate decreased with increasing crystallinity of the Mn oxides whether or not Fe(II) was present.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Exploring the Spatial Control of Topotactic Phase Transitions Using Vertically Oriented Epitaxial Interfaces

Engineering oxygen vacancy formation and distribution is a powerful route for controlling the oxygen sublattice evolution that affects diverse functional behavior. The controlling of the oxygen vacancy formation process is particularly important for inducing topotactic phase transitions that occur by transformation of the oxygen sublattice. Here we demonstrate an epitaxial nanocomposite approach for exploring the spatial control of topotactic phase transition from a pristine perovskite phase to an oxygen vacancy-ordered brownmillerite (BM) phase in a model oxide La 0.7 Sr 0.3 MnO 3 (LSMO). Incorporating a minority phase NiO in LSMO films creates ultrahigh density of vertically aligned epitaxial interfaces that strongly influence the oxygen vacancy formation and distribution in LSMO. Combined structural characterizations reveal strong interactions between NiO and LSMO across the epitaxial interfaces leading to a topotactic phase transition in LSMO accompanied by significant morphology evolution in NiO. Using the NiO nominal ratio as a single control parameter, we obtain intermediate topotactic nanostructures with distinct distribution of the transformed LSMO-BM phase, which enables systematic tuning of magnetic and electrical transport properties. The use of self-assembled heterostructure interfaces by the epitaxial nanocomposite platform enables more versatile design of topotactic phase structures and correlated functionalities that are sensitive to oxygen vacancies.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Steam oxidation and microstructural evolution of rare earth silicate environmental barrier coatings

A primary failure mode for environmental barrier coatings (EBCs) on SiC ceramic matrix composites (CMCs) is the oxidation of the intermediate Si-bond coating, where the formation of SiO 2 at the bond coating–EBC interface results in debonding and spallation. This work compares the microstructure evolution and steam oxidation kinetics of the Si-bond coating beneath yttrium/ytterbium disilicate ((Y/Yb)DS) and ytterbium disilicate/monosilicate (YbDS/YbMS) EBCs to better understand the impact of EBC composition on oxidation kinetics. After 500 1-h cycles at 1350°C, (Y/Yb)DS displayed a decreasing concentration of the monosilicate minor phase and increasing concentration of porosity as furnace cycling time increased, whereas the YbDS/YbMS EBC displayed negligible microstructural evolution. For both EBC systems, thermally grown oxide growth rates in steam were found to increase by approximately an order magnitude compared to dry air oxidation. The (Y/Yb)DS EBC displayed a reduced steam oxidation rate compared to YbDS/YbMS.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Damage free Ar ion plasma surface treatment on In{sub 0.53}Ga{sub 0.47}As-on-silicon metal-oxide-semiconductor device

In this paper, we investigated the effect of in-situ Ar ion plasma surface pre-treatment in order to improve the interface properties of In{sub 0.53}Ga{sub 0.47}As for high-κ top-gate oxide deposition. X-ray photoelectron spectroscopy (XPS) and metal-oxide-semiconductor capacitors (MOSCAPs) demonstrate that Ar ion treatment removes the native oxide on In{sub 0.53}Ga{sub 0.47}As. The XPS spectra of Ar treated In{sub 0.53}Ga{sub 0.47}As show a decrease in the AsO{sub x} and GaO{sub x} signal intensities, and the MOSCAPs show higher accumulation capacitance (C{sub acc}), along with reduced frequency dispersion. In addition, Ar treatment is found to suppress the interface trap density (D{sub it}), which thereby led to a reduction in the threshold voltage (V{sub th}) degradation during constant voltage stress and relaxation. These results outline the potential of surface treatment for III-V channel metal-oxide-semiconductor devices and application to non-planar device process.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Oxyfluoride superlattices K Ta O 3 / K M F 3 ( M = Zn , Ni ) : Structural and electronic phenomena

The structural and electronic properties of KTaO 3 /KZnF 3 and KTaO 3 /KNiF 3 oxyfluoride superlattices are studied from first-principles density functional theory calculations. Here we highlight, that beyond a critical layer thickness, these systems exhibit an insulator to metal transition that gives rise to the appearance of two-dimensional electron and hole gas, confined both, due the band alignment, within the oxide layer. The origin of the insulator to metal transition is related to the polar discontinuity at the interfaces. The behavior is discussed in terms of a simple electrostatic model and compared to that of the prototypical LaAlO 3 /SrTiO 3 oxide system. The magnetic properties KTaO 3 /KNiF 3 superlattices are further discussed, revealing a sizable Rashba-type spin splitting at these interfaces, much larger than in similar oxide/oxide systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Dynamic Atom Clusters on AuCu Nanoparticle Surface During CO Oxidation

Supported alloy nanoparticles are prevailing alternative low-cost catalysts for both heterogeneous and electrochemical catalytic processes. Selective interaction of an alloy component with a specific reactant induces a dynamic structural change of alloy nanoparticles under reaction conditions and largely controls their catalytic properties. However, such a multi-component dynamic-interaction-controlled evolution, both structural and chemical, remains far from clear. Herein, by using state-of-the-art environmental TEM, we directly visualize, in-situ at the atomic scale, the evolution of an AuCu alloy nanoparticle supported on CeO2 during CO oxidation. We find that gas molecules can “free” metal atoms on the {001} surface and form highly mobile atom clusters. Remarkably, we discover that CO exposure induces Au segregation and activation on nanoparticle surface, while O2 exposure leads to the segregation and oxidation of Cu on the particle surface. The as-formed Cu2O/AuCu interface may facilitate CO-O interaction corroborated by DFT calculations. These findings provide insights into the atomistic mechanisms on alloy nanoparticles during catalytic CO oxidation reaction, and to a broad scope of rational design of alloy nanoparticle catalysts.

Luo, Langli↗
DOE OSTI2018

Experimental Investigation of the System Nonmetallic Inclusion-Molten Steel-Refractory System at High Temperatures

In continuous casting, the buildup of particles in the flow control system is called clogging. Clogging may be caused by preexisting nonmetallic inclusions attached to the refractory wall, interrupting or disturbing the fluid flow conditions. The interfacial properties of the nonmetallic inclusion-molten steel-refractory system are believed to play a key role in the clogging tendency of selected steel grades in contact with ceramics. The combination of a high-temperature laser scanning confocal microscope (HT-LSCM), which is employed for in situ observation of inclusions’ behavior in the liquid steel or at the steel/ceramic interface, and a high-temperature drop shape analyzer (HT-DSA), which is utilized for contact angle measurements, enabled the investigation of reactions and interactions in the described system at experimental temperatures of approximately 1600 °C. The objective of this study was to develop a method to obtain qualitative information on the deposition of nonmetallic inclusions (NMIs) in the submerged entry nozzle (SEN) during casting. The behavior of solid NMIs from a calcium-treated steel at the interface between liquid steel and Al{sub 2}O{sub 3} or MgO was investigated by means of HT-LSCM. During the test evaluation, inclusions were found to separate at the steel/ceramic interface due to an attractive force. This force increased with the NMIs’ size as well as with the ceramic particle change. The HT-DSA provided contact angle data between molten steel and the ceramic used for implementation into the theoretical capillary force model. A clear correspondence between the estimated and calculated capillary forces was observed. The change in the wettability of the system, as induced by the change in the ceramic from MgO to Al{sub 2}O{sub 3}, showed a reduction of the attractive forces, indicating a weaker separation toward the SEN for the same inclusion type. This study confirmed the strong potential of wettability investigations to predict the clogging tendency of certain steel grades.

36 MATERIALS SCIENCE↗
DOE OSTI2023

The Dynamics of Oxygen Ion Exchange in Epitaxial Strontium Cobaltite Bilayers

Abstract The exchange of ions across interfaces is key to the field of iontronics, where the properties of the device can be altered by the local ion concentration. This study investigates a complex oxide system where structural and electronic phase transitions can be driven by changes in the concentration of oxygen ions. In situ coherent X‐ray studies are conducted on epitaxial bilayers of insulating SrCoO 2.5 and metallic SrCoO 3 − δ . The diffusion of oxygen ions across the bilayer is studied with X‐ray photon correlation spectroscopy to capture the dynamical behavior of the interface in reducing and oxidizing environments. The behavior is strongly asymmetric, with much slower dynamics appearing in reducing versus oxidizing environments. According to the correlation functions determined from different points in reciprocal space, this study finds that the dynamics near the center of the SrCoO 2.5 crystal are generally similar to those near the heterointerfaces. The results suggest that the interface is stable and reversible, making SrCoO x a model system for the study of iontronic behavior.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Intermittent oxidation kinetics and metal/oxide interfacial undulation

The phenomenon of oxygen-adsorption-induced surface restructuring is widespread across various metal-oxygen systems, yet its impact on initiating bulk oxide formation remains largely unexplored. In this study, through in-situ atomic-resolution electron microscopy observations of surface oxidation of Cu(110) and Cu 85 Au 15 (110), we unveil intermittent oxide-film growth modulated by oxygen-induced surface restructuring. This modulation is evidenced by repeated pinning of Cu 2 O growth front at isolated Cu columns of the c(6×2)-O reconstruction, owing to required long-range diffusion of Cu and O atoms to the Cu 2 O growth front. We reveal that Cu vacancies, generated at the Cu 2 O growth front, are injected into the Cu 2 O/Cu interface, inducing hill-and-valley undulation of the Cu 2 O film. In contrast, atomic vacancies produced during the Cu 85 Au 15 (110) oxidation preferentially migrate into interfaces between Au-rich and Au-poor regions in the bulk, resulting in a flat and adherent Cu 2 O film. These findings demonstrate the critical role of oxygen-induced surface restructuring in modulating oxide-film-growth kinetics and manipulability of the fate of injected vacancies by alloying, thereby offering insights applicable to a broader range of metal-oxygen systems for fine-tuning oxidation kinetics and enhancing oxide/metal interfacial adhesion.

25 ENERGY STORAGE↗
DOE OSTI2014

Nitride passivation reduces interfacial traps in atomic-layer-deposited Al{sub 2}O{sub 3}/GaAs (001) metal-oxide-semiconductor capacitors using atmospheric metal-organic chemical vapor deposition

Using an atmospheric metal-organic chemical vapor deposition system, we passivated GaAs with AlN prior to atomic layer deposition of Al{sub 2}O{sub 3}. This AlN passivation incorporated nitrogen at the Al{sub 2}O{sub 3}/GaAs interface, improving the capacitance-voltage (C–V) characteristics of the resultant metal-oxide-semiconductor capacitors (MOSCAPs). The C–V curves of these devices showed a remarkable reduction in the frequency dispersion of the accumulation capacitance. Using the conductance method at various temperatures, we extracted the interfacial density of states (D{sub it}). The D{sub it} was reduced over the entire GaAs band gap. In particular, these devices exhibited D{sub it} around the midgap of less than 4 × 10{sup 12} cm{sup −2}eV{sup −1}, showing that AlN passivation effectively reduced interfacial traps in the MOS structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Multiphase layered transition metal oxide positive electrodes for sodium ion batteries

Abstract Multiphase layered transition metal oxides (LTMOs) for sodium ion battery (SIB) positive electrodes with phase interfaces across multiple length scales are a promising avenue toward practical, high‐performance SIBs. Combinations of phases can complement each other's strengths and mitigate their weaknesses if their interfaces are carefully controlled. Intra‐ and interparticle phase interactions from nanoscale to macroscale must be carefully tuned to generate distinct effects on properties and performance. An informed design strategy must be paired with relevant synthesis techniques and complemented by spatially resolved characterization tools to manipulate different length scales and interfaces. This review examines the design, synthesis, and characterization strategies that have been demonstrated for the preparation of heterogeneous, multiphasic LTMOs with phase interfaces across varied length scales.

25 ENERGY STORAGE↗
DOE OSTI2014

Impact of environmental conditions on the chemical surface properties of Cu(In,Ga)(S,Se){sub 2} thin-film solar cell absorbers

Environmentally driven aging effects play a crucial role in thin-film solar cells based on Cu(In,Ga)(S,Se){sub 2}, both for long-term stability and short air exposure during production. For a better understanding of such effects, Cu(In,Ga)(S,Se){sub 2} absorber surfaces were investigated by x-ray photoelectron and Auger electron spectroscopy after exposure to different environmental conditions. Identical absorbers were stored in a nitrogen atmosphere, in damp heat, and under ambient conditions for up to 14 days. We find varying degrees of diffusion of sulfur, copper, and sodium towards the surface, with potential impact on the electronic surface structure (band gap) and the properties of the interface to a buffer layer in a solar cell device. Furthermore, we observe an oxidation (in decreasing order) of indium, copper, and selenium (but no oxidation of sulfur). And finally, varying amounts of carbon- and oxygen-containing adsorbates are found. In particular, the findings suggest that, for ambient air exposure, sodium carbonate is formed at the surface.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Improved Mechanical Adhesion Enabled by Bilayer Hole Transport Layers in Perovskite Solar Cells

We investigate buried-interface mechanical integrity in metal-halide perovskite half-cells using carbazole-based self-assembled monolayers (SAMs) and bilayer hole transport layers composed of metal oxide underlayers and SAMs. Combining macroscopic and microscopic characterization, we quantify interfacial fracture energy (Gc) and identify delamination pathways. SAM deposition on NiOx increases Gc by approximately 10-fold compared with ITO, highlighting the importance of oxide underlayer selection. To isolate the effect of removing noncovalently bonded SAM molecules, we use vapor-deposited perovskite as a solvent-free model system, decoupling the rinse step inherent to solution processing. Removing loosely bound SAM molecules increases Gc from 0.94 +- 0.08 J.m-2 to 4.82 +- 0.79 J.m-2. These results show that oxide selection and SAM binding quality are both critical for strengthening buried interfaces and designing mechanically robust perovskite solar cells.

14 SOLAR ENERGY↗
DOE OSTI2014

Strain-modulated antiferromagnetic spin orientation and exchange coupling in Fe/CoO(001)

The effect of CoO spin orientation on exchange coupling was investigated in single-crystalline Fe/CoO/MnO/MgO(001) systems. An antiferromagnetic CoO spin reorientation transition from the in-plane direction to the out-of-plane direction was found to be associated with the in-plane strain transition in CoO film from compression to expansion. The induced uniaxial anisotropies by exchange coupling at the Fe/CoO interface are significantly stronger for the in-plane CoO spin orientation than for the out-of-plane CoO spin orientation. Our study provides a way to modify the exchange coupling in the ferromagnetic (FM)/antiferromagnetic (AFM) bilayer by modulating the strain in the AFM film.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Ultra-thin distributed Bragg reflectors via stacked single-crystal silicon nanomembranes

In this paper, we report ultra-thin distributed Bragg reflectors (DBRs) via stacked single-crystal silicon (Si) nanomembranes (NMs). Mesh hole-free single-crystal Si NMs were released from a Si-on-insulator substrate and transferred to quartz and Si substrates. Thermal oxidation was applied to the transferred Si NM to form high-quality SiO{sub 2} and thus a Si/SiO{sub 2} pair with uniform and precisely controlled thicknesses. The Si/SiO{sub 2} layers, as smooth as epitaxial grown layers, minimize scattering loss at the interface and in between the layers. As a result, a reflection of 99.8% at the wavelength range from 1350 nm to 1650 nm can be measured from a 2.5-pair DBR on a quartz substrate and 3-pair DBR on a Si substrate with thickness of 0.87 μm and 1.14 μm, respectively. The high reflection, ultra-thin DBRs developed here, which can be applied to almost any devices and materials, holds potential for application in high performance optoelectronic devices and photonics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Resolving Iron(II) Sorption and Oxidative Growth on Hematite (001) Using Atom Probe Tomography

The distribution of iron resulting from the autocatalytic interaction of aqueous Fe(II) with the hematite (001) surface was directly mapped in three dimensions (3D) for the first time, using iron isotopic labelling and atom probe tomography (APT). Analyses of the mass spectrum showed that natural abundance ratios in 56Fe-dominant hematite are recovered at depth with good accuracy, whereas at the relict interface with 57Fe(II) solution evidence for hematite growth by oxidative adsorption of Fe(II) was found. 3D reconstructions of the isotope positions along the surface normal direction showed a zone enriched in 57Fe, which was consistent with an average net adsorption of 3.2 – 4.3 57Fe atoms nm–2. Statistical analyses utilizing grid-based frequency distribution analyses show a heterogeneous, non-random distribution of oxidized Fe on the (001) surface, consistent with Volmer-Weber-like island growth. The unique 3D nature of the APT data provides an unprecedented means to quantify the atomic-scale distribution of sorbed 57Fe atoms and the extent of segregation on the hematite surface. This new ability to spatially map growth on single crystal faces at the atomic scale will enable resolution to long-standing unanswered questions about the underlying mechanisms for electron and atom exchange involved in a wide variety of redox-catalyzed processes at this archetypal and broadly relevant interface.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2015

Role of the (Ta/Nb)O{sub x}/Al{sub 2}O{sub 3} interface on the flatband voltage shift for Al{sub 2}O{sub 3}/(Ta/Nb)O{sub x}/Al{sub 2}O{sub 3} multilayer charge trap capacitors

The authors studied the characteristics of Si/Al{sub 2}O{sub 3}/(Ta/Nb)O{sub x}/Al{sub 2}O{sub 3}/SiO{sub 2}/Pt charge trap capacitors fabricated by atomic layer deposition and postmetallization annealing at 400 °C. Al{sub 2}O{sub 3} and (Ta/Nb)O{sub x} films are amorphous and have negligible fixed charges. In program mode, a flatband voltage (V{sub fb}) drastically shifts toward the positive direction at a short program time of 10{sup −4} s. A large V{sub fb} shift of approximately 4 V arises after programming at 1 mC/cm{sup 2} because there is a large difference in the conduction band offset between the (Ta/Nb)O{sub x}-charge trapping layer (TNO-CTL) and the Al{sub 2}O{sub 3}-blocking layer (AlO-BL) (1.8 eV). In the retention mode, most of the trapped electrons in the TNO-CTL transfers across the Al{sub 2}O{sub 3}-tunneling layer (AlO-TL) rather than the AlO-BL. The thickness of the AlO-TL affects the V{sub fb} shift degradation behavior in the retention mode. The injected electrons are dominantly located at the TNO-CTL/ALO-BL interface, determined from the thickness dependence of the TNO-CTL on the V{sub fb} shift.

42 ENGINEERING↗
DOE OSTI2013

Fe3-xTixO4 Nanoparticles as Tunable Probes of Microbial Metal Oxidation

Present and emerging biotechnological applications for iron (oxyhydr)oxide nanomaterials depend on their interaction with microorganisms, as do their toxicity, transport, and fate in biological and environmental systems. However, mass or electron transfer along key molecular pathways at microbe-nanomaterial interfaces is extremely difficult to quantify because of system complexity. Inspired by Fe(II)-oxidizing microbes widespread in nature, we isolate and characterize one such pathway by examining the oxidation of Fe3-xTixO4 (magnetite-titanomagnetite) nanoparticles by the bacterial electron transfer enzyme MtoA, a decaheme c-type cytochrome. Oxidation by MtoA was studied as a function of the thermodynamic driving force for electron transfer by controlling the Ti(IV) doping content (x), which tunes the solid-state Fe(II)/Fe(III) ratio built into the nanoparticles. A higher Fe(II)/Fe(III) ratio appears to proportionally increase the electron transfer kinetics to the cytochrome. In situ x-ray diffraction indicated that during oxidation the spinel ferrite lattice remains intact while structural Fe(II) is progressively depleted. Surface and atomic site specific Fe L2,3-edge x-ray magnetic circular dichroism indicated that MtoA directly accesses magnetically-ordered B-sublattice Fe(II) at the interface. This study provides first quantitative insights into an isolated molecular pathway for biotransformation of iron (oxyhydr)oxide nanomaterials. And, more generally, it also illustrates new techniques for probing these pathways in detail, featuring use of tailored nanoparticles, purified metalloenzyme, and synchrotron x-ray absorption spectroscopies.

magnetite ∙ titanomagnetite ∙ microbial iron oxida↗